X-ray based computed tomography, abbreviated to CT, is an established method of three-dimensional imaging, especially for diagnostic purposes. CT not only makes possible spatially high-resolution imaging, but also allows, by way of a timed series of recordings, especially with administration of contrast medium, detailed conclusions to be drawn about the function of organs, for example the heart and the surrounding coronary vessels. In order to optimize the conditions for a CT recording as well as reconstruction of such recordings, regular patient-dependent information is needed. Such information is obtained for example from an electrocardiogram, abbreviated to EKG. Basically a dose which is as low as possible must be ensured for the patient in order to prevent damage from x-ray radiation. Therefore patient-dependent information is needed, especially for reduction of the applied dose. The dose is understood as the energy dose for example, i.e. the energy of the x-ray radiation that a patient absorbs per kg of body weight; the Gray unit is used for a dose defined in this way. A dose can also be understood as the effective dose in Sievert units.
Patent DE 102 45 943 B4 discloses a method and a CT device for creating CT images of an organ which moves periodically. The patent relates to the creation of CT images of a periodically moving organ of a living being, wherein the organ has areas with rest and movement phases and the rest phases of different areas of the organ occur at different points in time, by way of a CT device with an x-ray radiation source moved around the body of the living being to be examined for creating the CT images. First of all a plurality of projections for the purposes of image creation is recorded during at least one orbit of the x-ray radiation source around the object to be examined and a period of time, which is at least equal to a period of the movement, is recorded. Then the data corresponding to the projections is analyzed as to whether it was obtained during a rest or movement phase of a region of interest of the organ in each case. Then an image of the organ is reconstructed using only such data as was obtained during a rest phase of the respective region of interest. The analyzing of the data corresponding to the projections includes it being analyzed in relation to a number of regions of interest of the organ as to whether a rest or a movement phase of the respective area was obtained, wherein, in relation to all regions of interest of the organ, in each case an image of the organ is reconstructed using only such data as has been obtained during a rest phase of the respective region of interest of the organ, and wherein an image comprising all regions of interest of the organ in their respective rest phase is created from the images representing the rest phase of the individual regions of interest of the organ. With the CT images necessarily reconstructed in different heart phases an overall diagnosis for all vessels on the basis of the images showing the individual vessels in their rest phase is possible. The heart signals are used for an improved reconstruction, but the method does not directly reduce the applied dose. A signal representing a threshold criterion for the physiological function can actually be predetermined which, when fulfilled, is a region of interest of the organ in a rest phase in each case. To record the projections necessary for creating an image the x-ray radiation source is only activated during those time segments during which the threshold criterion is fulfilled. This method can be realized as EKG-triggered dose modulation and lowers the applied dose directly. However the projections recorded then no longer allow a reconstruction in each phase of the heart cycle, since only projections from the rest phase are available.